Transfer printing processes are suitable for printing on materials such as textiles or rigid bodies, which for mechanical reasons are difficult to print on by direct printing methods. In the transfer printing method, an image is first printed on a flexible sheet-like transfer material and this printed image is subsequently transferred from the said transfer material onto the final object for which the print image is intended. A specific embodiment for this kind of transfer printing process is the dye sublimation process which, for instance, is described on page 468 in B. Thompson's “Printing Materials—Science and Technology” (1998). In this process, the image to be printed is applied to the transfer material using printing inks which, after the print has dried, are exposed to heat and allowed to evaporate such that the vapors are deposited as an image on the final material. Digital printing, in particular the inkjet printing process, is a convenient process that can be used for applying sublimation inks to the transfer material. A major benefit of this process is that it facilitates the printing of personalized images, for instance, on textiles.
Printing inks for inkjet printing with dyestuffs that are transferred through sublimation onto the final substrate are, for instance, described in DE 102 46 209 A1.
Paper is the preferred transfer material that is used in the first step in the inkjet printing technique. EP 1 101 682 A1 describes a type of coated paper that is characterized by a low level of air permeability on the side that is to be printed on. The purpose of this type of paper is to prevent the penetration of sublimable dyes into the porous interior of the paper and the consequent loss thereof during transfer onto the final substrate material. However, such types of paper with low porosity on the side that is to be printed on absorb the inkjet inks very slowly. As a result—and especially at high printing speeds—the ink dries very slowly, it can smudge on the surface and the printing resolution can thus be unsatisfactory.
In US 2008/229962 A1, therefore, a transfer paper coating that contains silica and a comparatively low amount of binders was proposed. Such a coating exhibits a significantly higher level of air permeability. This allows the ink to be absorbed but prevents a loss of sublimable dyes in the interior of the paper when transferring the image onto the final substrate material.
So-called “tacky” (adhesive) transfer papers refer to products that adhere to substrates such as textiles when transferred by means of the sublimation process. These products are especially used for stretch fabrics (for instance, knitted fabrics) to prevent the formation of shadows (ghosting) and to reduce the number of rejects during textile printing. Typical applications include textile printing for sportswear. The purpose of this type of adhesion is thus to prevent a displacement of the printing medium with respect to the receiving material arranged on the printing medium and vice versa.
Such types of adhesive transfer paper with swellable, non-porous layers are described in EP 1 102 682 A1 and EP 1 878 829 A1. A disadvantage of such types of paper is that they require a very long time to dry.
A so-called thermal transfer (or thermal sublimation) paper is also described in DE 10 2014 116550 A1 in which the use of thermoplastic particles with a melting point of 35° C. to 150° C. and an average particle size of 0.3 μm to 5 μm in the thermal transfer layers is proposed. The aim of using thermoplastic particles is to optimize the adhesion of the thermal sublimation paper when printing on flat textiles. The thermal transfer layer described in this document has a binder content of 55% to 80% (dry weight) and may even contain pigments. Despite the presence of pigments, the binder content in this proportion ensures a sealed, film-like thermal transfer layer that is not porous. The disadvantage of the technique using the materials described above is that—in contrast to microporous layers—the rate of drying is significantly lower. Moreover, the thermoplastic particle content has to be rather high in order to ensure an appreciable level of adhesion in the first place. This can greatly impact the print quality (line sharpness) and the transfer quality (optical densities on textiles). Apart from this, it is not possible to independently control the level of adhesion on textile substrates and the print and transfer quality with this technique.